[turbofan] Introduce CheckString simplified operator.

src/compiler/js-native-context-specialization.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/js-native-context-specialization.h"
 
 #include "src/accessors.h"
 #include "src/code-factory.h"
 #include "src/compilation-dependencies.h"
 #include "src/compiler/access-builder.h"
@@ skipping 136 matching lines @@
   for (size_t j = 0; j < access_infos.size(); ++j) {
     PropertyAccessInfo const& access_info = access_infos[j];
     Node* this_value = value;
     Node* this_receiver = receiver;
     Node* this_effect = effect;
     Node* this_control;
 
     // Perform map check on {receiver}.
     Type* receiver_type = access_info.receiver_type();
     if (receiver_type->Is(Type::String())) {
-      Node* check = graph()->NewNode(simplified()->ObjectIsString(), receiver);
       if (j == access_infos.size() - 1) {
-        this_effect = graph()->NewNode(simplified()->CheckIf(), check,
-                                       this_effect, fallthrough_control);
+        this_receiver = this_effect =
+            graph()->NewNode(simplified()->CheckString(), receiver, this_effect,
+                             fallthrough_control);
         this_control = fallthrough_control;
         fallthrough_control = nullptr;
       } else {
+        Node* check =
+            graph()->NewNode(simplified()->ObjectIsString(), receiver);
         Node* branch =
             graph()->NewNode(common()->Branch(), check, fallthrough_control);
         fallthrough_control = graph()->NewNode(common()->IfFalse(), branch);
         this_control = graph()->NewNode(common()->IfTrue(), branch);
       }
     } else {
       // Emit a (sequence of) map checks for other {receiver}s.
       ZoneVector<Node*> this_controls(zone());
       ZoneVector<Node*> this_effects(zone());
       int num_classes = access_info.receiver_type()->NumClasses();
@@ skipping 860 matching lines @@
 }
 
 
 SimplifiedOperatorBuilder* JSNativeContextSpecialization::simplified() const {
   return jsgraph()->simplified();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8